The disclosure of Japanese Patent Application No. 2001-305424 filed on Oct. 1, 2001 including the specification, drawings and abstract is incorporated herein by reference in its entirety.
1. Field of the Invention
The invention relates to an exhaust gas purifying apparatus for an internal combustion engine and a control method thereof.
2. Description of the Related Art
As an exhaust gas purifying apparatus for purifying an exhaust gas in an internal combustion engine, there is known an art in which plural catalysts for purifying the exhaust gas are arranged in parallel in exhaust passages. In such an exhaust gas purifying apparatus for an internal combustion engine, catalysts having an oxidation function such as nitrogen oxides (NOx) catalysts may be arranged in parallel.
The NOx catalyst can purify the exhaust gas discharged from an internal combustion engine such as a diesel engine and a lean burn gasoline engine that can operate in a lean burn state. The NOx catalyst includes, for example, a selective reduction type NOx catalyst and an occlusion-reduction type NOx catalyst.
The occlusion-reduction type NOx catalyst performs an absorbing and emitting operation. That is, the occlusion-reducion type NOx catalyst absorbs nitrogen oxides (NOx) when an oxygen concentration of the inflowing exhaust gas is high, and emits the absorbed NOx and reduces the NOx into nitrogen (N2) when the oxygen concentration of the inflowing exhaust gas is low.
In the case of the occlusion-reduction type NOx catalyst, an air-fuel ratio of the exhaust gas in the internal combustion engine is lean during a normal operation, so that the NOx in the exhaust gas is absorbed in the NOx catalyst. However, if the exhaust gas at a lean air-fuel ratio continues to be supplied to the NOx catalyst, the amount of the NOx absorbed in the NOx catalyst reaches a saturation amount, and as a result, the catalyst cannot absorb the NOx any further and the NOx passes through the catalyst. Thus, it is required that, in the NOx catalyst, the oxygen concentration of the exhaust gas should be decreased at predetermined timing before the amount of the NOx absorbed in the NOx catalyst reaches a saturation amount, and a component amount of hydrogen carbide (HC) in the exhaust gas should be increased, thereby emitting the NOx absorbed in the NOx catalyst and reducing the NOx into nitrogen (N2) in order to recover the NOx absorbing ability of the NOx catalyst.
Thus, in the exhaust gas purifying apparatus using a lean NOx catalyst, the oxygen concentration of the exhaust gas needs to be decreased intermittently for purifying the NOx. As an example of a method of decreasing the oxygen concentration of the exhaust gas intermittently, a fuel supply into the exhaust gas can be adopted.
Sulfur oxides (SOx) produced by burning of a sulfur component, which is contained in the fuel, is also absorbed in the occlusion-reduction type NOx catalyst according to the same mechanism as in the case of NOx. The SOx thus absorbed in the catalyst is not easily emitted as compared to NOx, and is accumulated in the NOx catalyst. This is called sulfur poisoning (SOx poisoning). In the SOx poisoning, NOx purification efficiency is lowered. Therefore, a process for recovering the catalyst from the SOx poisoning (SOx poisoning recovery process) needs to be performed at appropriate timing. The SOx poisoning recovery process is performed by setting the NOx catalyst at a high temperature (600 to 650 degrees, for example) while passing the exhaust gas with a reduced low oxygen concentration in the NOx catalyst.
However, since the temperature of the exhaust gas is low during the lean burn operation of the engine, it is difficult to raise the temperature of the catalyst to a temperature range in which the catalyst can be recovered from the SOx poisoning.
In order to solve such a problem, for example, Japanese Patent No. 2727906 discloses an exhaust gas purifying apparatus for an internal combustion engine in which two particulate filters supporting an NOx absorbent are arranged in parallel in an exhaust passage. In the device, the exhaust gas flowing into the NOx absorbent of either one of the particular filters is blocked, i.e., the particular filters are blocked one by one, so that the NOx is emitted from the blocked NOx absorbent. According to the exhaust gas purifying apparatus for an internal combustion engine, when one of the NOx absorbents is performing a regeneration operation, the internal engine can be operated by passing the exhaust gas through the other of the NOx absorbents. Thus, a total flow of the exhaust gas does not need to be reduced and an output of the internal combustion engine does not decrease. Therefore, the regeneration operation of the NOx absorbent can be performed at appropriate timing irrespective of the operation condition of the engine. Furthermore, it is possible to raise the temperature of the filter up to a temperature required for recovery from the SOx poisoning by burning the particulates when the reducing agent is supplied.
Each catalyst has its temperature range in which the exhaust gas is effectively purified (temperature window). Thus, it is important to raise the temperature to each temperature window as quickly as possible.
In the device disclosed in Japanese Patent No. 2727906, almost no exhaust gas flows in the NOx absorbent when the reducing agent is supplied, so that the NOx absorbent can hardly obtain energy from the exhaust gas. At this time, if the NOx absorbent can obtain much energy from the exhaust gas, the temperature of the NOx absorbent can be raised promptly.
Meanwhile, if the temperature of the NOx absorbent is raised by raising the temperature of the exhaust gas, smoke or deterioration of the operation state of the engine may occur. In addition, if the temperature of the NOx absorbent is raised by supplying the reducing agent into the exhaust gas, the oxygen may be insufficient depending on the supplying amount of the reducing agent. In this case, a part of the reducing agent cannot be oxidized, and accordingly, the temperature cannot be raised sufficiently.
Above-mentioned heating control for the catalyst (heating control) is performed also in the case where the particulates accumulated in the NOx absorbent are oxidized, and the temperature of the catalyst needs to be raised promptly.
The invention is made in order to solve the above-mentioned problem. Accordingly, an object of the invention is to provide an art for raising the temperature of plural catalysts promptly in an exhaust gas purifying apparatus for an internal combustion engine including the plural catalysts having an oxidation function.
According to a first aspect of the invention, an exhaust gas purifying apparatus for an internal combustion engine includes plural exhaust passages of the internal combustion engine which are provided in parallel, catalysts (a first catalyst and a second catalyst) which have an oxidation function and are provided in the plural exhaust passages respectively, a reducing agent supply device that supplys a reducing agent to the catalysts, a catalyst heating device that raises temperatures of the catalysts independently of the reducing agent supply device, a flow amount adjusting device that adjusts an amount of the exhaust gas flowing in the exhaust passages, and a temperature estimating device that estimates the temperatures of the catalysts. Further, the exhaust gas purifying apparatus for an internal combustion engine according to the first aspect of the invention includes a control device. In a case where the temperatures of the catalysts need to be raised, the control device selects a catalyst (the first catalyst) in which the exhaust gas flows, and allows a larger amount of the exhaust gas to flow in the selected catalyst than in the other catalysts (e.g. the second catalyst) by the flow amount adjusting device. In addition, the control device raises the temperature of the selected catalyst by the catalyst heating device when the temperatures of the catalysts are lower than a temperature range in which the reducing agent can be purified, and supplies the reducing agent to the selected catalyst to raise the temperature of the selected catalyst by the reducing agent supply device when the temperatures of the catalysts are in the temperature range in which the reducing agent can be purified.
According to the first aspect of the invention, energy of the exhaust gas can be concentrated on the selected catalyst (the first catalyst) and the purification efficiency can be increased in a short time by allowing the exhaust gas to flow in only the selected catalyst. In addition, the selected catalyst can be heated promptly and then the other catalysts (e.g. the second catalyst) can be heated easily.
According to the first aspect, the exhaust gas is allowed to flow in the selected catalyst by the flow amount adjusting device, the temperature of the selected catalyst is raised by the catalyst heating device when the temperature of the catalysts is lower than the temperature range in which the reducing agent can be purified. In this case, the catalyst heating device raises the temperature of the exhaust gas, for example. When the temperature of the catalysts reaches the temperature range in which the reducing agent can be purified, the reducing agent is supplied to the selected catalyst by the reducing agent supply device for heating the selected catalyst to a required temperature (temperature at which the NOx can be purified, for example). Accordingly, the selected catalyst can be promptly heated.
The temperature estimating device may measure the temperature of the catalyst directly or estimate the temperature of the catalyst by measuring the temperature of the exhaust gas upstream of the catalyst.
According to the first aspect, the control device may allow all the exhaust gas to flow in the selected catalyst and cut off the flow of the exhaust gas into the catalysts which are not selected.
In addition, according to the first aspect, when the temperature of the selected catalyst is raised to a temperature range in which nitrogen oxides can be purified, a new catalyst may be selected from the catalysts that have not been selected so that the exhaust gas flows in the newly selected catalyst. By heating the catalysts in turn as mentioned above, at least one of the catalysts can be promptly activated.
Further, according to the first aspect, the catalysts may be supported on a particulate filter that can temporarily trap particulate matters in the exhaust gas and when the temperature of the selected catalyst is heated to a temperature range in which the particulate matter can be removed, a new catalyst may be selected from the catalysts that have not been selected so that the exhaust gas flows in the newly selected catalyst. By heating the catalysts in turn as mentioned above, at least one of the catalysts can be promptly activated to the temperature range in which the particulate matter can be removed.
Furthermore, according to the first aspect, the amount of the exhaust gas flowing in the catalysts which are not selected may be increased if a pressure of the exhaust gas upstream of the catalysts is equal to or greater than a predetermined value.
The exhaust gas flows in all the catalysts during a normal operation of the engine, so that a capacity of each catalyst can be made small. However, in the exhaust gas purifying apparatus for an internal combustion engine, if the exhaust gas is concentrated to flow in the selected catalyst, an amount of the generated exhaust gas becomes greater than the amount of the exhaust gas which can flow in the catalyst. As a result, the pressure of the exhaust gas upstream of the catalysts may be increased. Therefore, in this case, the pressure of the exhaust gas upstream of the catalysts can be decreased by allowing the exhaust gas to flow in the other catalysts.
According to a second aspect of the invention, an exhaust gas purifying apparatus for an internal combustion engine includes a NOx catalyst which have an oxidation function and are provided in an exhaust passage of internal combustion engine, a reducing agent supply device that supplies a reducing agent to the NOx catalyst, an NOx catalyst heating device that raises a temperature of the NOx catalyst independently of the reducing agent supply device, a flow amount adjusting device that adjusts an amount of the exhaust gas flowing in the exhaust passage, and a temperature estimating device that estimates the temperature of the NOx catalyst. Further, according to the second aspect of the invention, when the temperature of the NOx catalyst are lower than a temperature range in which the reducing agent can be purified, the flow amount adjusting device allows a larger amount of exhaust gas to flow in the NOx catalyst than when the NOx is reduced in the NOx catalyst, and the NOx catalyst heating device raises the temperature of the NOx catalyst When the temperatures of the NOx catalyst are in the temperature range in which the reducing agent can be purified, the reducing agent supply device supplies the reducing agent so that an air-fuel ratio of the exhaust gas becomes greater than a stoichiometric air-fuel ratio to raise the temperature of the NOx catalyst.
According to the second aspect of the invention, the NOx catalyst to be heated can be brought into a state appropriate for being heated by allowing a larger amount of the exhaust gas to flow in the NOx catalyst than when the NOx is reduced, and making the air-fuel ratio of the exhaust gas flowing in the NOx catalyst slightly larger than the stoichiometric air-fuel ratio.
In view of prevention of deterioration of fuel consumption when the NOx is emitted from the NOx catalyst, it is preferable that the amount of the exhaust gas flowing in the NOx catalyst be decreased and further the air-fuel ratio of the exhaust gas be made smaller than the stoichiometric air-fuel ratio. On the other hand, in view of prompt heating of the NOx catalyst, it is preferable that the exhaust gas flowing in the NOx catalyst be increased and further the air-fuel ratio of the exhaust gas be made larger than the stoichiometric air-fuel ratio.
According to the second aspect of the invention, it is possible to heat the NOx catalyst promptly by giving priority to the heating of the NOx catalyst.
The temperature estimating device may measure the temperature of the catalyst directly or estimate the temperature of the catalyst by measuring the temperature of the exhaust gas upstream of the catalyst.
According to a third aspect of the invention, in a control method of an internal combustion engine, in a case where the temperatures of the catalysts need to be raised, a catalyst (a first catalyst) in which an exhaust gas flows is selected and a larger amount of the exhaust gas is allowed to flow in the selected catalyst than in the other catalysts (e.g. a second catalyst). In addition, when the temperatures of the catalysts are lower than a temperature range in which a reducing agent can be purified, the temperature of the selected catalyst is raised. When the temperatures of the catalysts are in the temperature range in which the reducing agent can be purified, the reducing agent is supplied to the selected catalyst so as to raise the temperature thereof.
According to the third aspect, the energy of the exhaust gas can be concentrated on the selected catalyst (the first catalyst) and the purification efficiency can be increased in a short time by allowing the exhaust gas to flow in only the selected catalyst. In addition, the selected catalyst can be heated promptly and then the other catalysts (e.g. the second catalyst) can be heated easily.
According to a fourth aspect of the invention, in a method for purifying an exhaust gas of an internal combustion engine, when the temperature of the NOx catalyst are lower than a temperature range in which the reducing agent can be purified, a larger amount of the exhaust gas is allowed to flow in the NOx catalyst than when the NOx is oxidized in the NOx catalyst so as to raise the temperature of the NOx catalyst. When the temperature of the catalyst are in the temperature range in which the reducing agent can be purified, the reducing agent is supplied so that the air-fuel ratio of the exhaust gas becomes larger than the stoichiometric air-fuel ratio, thereby raising the temperature of the NOx catalyst.
According to the fourth aspect of the invention, the NOx catalyst to be heated can be brought into a state appropriate for being heated by allowing a larger amount of the exhaust gas to flow in the NOx catalyst than when the NOx is reduced, and making the air-fuel ratio of the exhaust gas flowing in the NOx catalyst slightly greater than the stoichiometric air-fuel ratio.